This invention relates to high fidelity audio amplifiers, particularly power amplifiers, and still more particularly, to means for minimizing or eliminating discontinuities and transitional gain variations in such amplifiers such as occur when the active amplifying devices are successively driven between their conductive and non-conductive states. Although useful overall signal amplification can be achieved, such discontinuities and transitional gain variations prevent the attainment of true high fidelity audio amplification and produce cross-over distortion. Thse problems commonly occur with active amplifying devices such as bipolar transistors, field effect transistors, and vacuum tubes.
A high fidelity audio amplifier operating as a Class A type amplifier does not experience these problems because the active amplifying devices in such amplifiers are not successively driven between conductive and non-conductive states during the normal amplifying operation. Nevertheless, Class A amplifiers are not generally used when a relatively high power output such as necessary to drive a high fidelity speaker system is required. The reason for this is that conventional Class A amplifiers have their active amplifying devices in conductive states at all times, which, in turn, means that such devices continuously dissipate power, thereby requiring special cooling and heat dissipating means. Also, conventional Class A amplifiers generally have low efficiency.
As a result, Class AB and Class B type amplifiers have previously been preferred when high output power is required, even though fidelity is compromised. For example, although a Class AB amplifier has an efficiency advantage relative to a Class A amplifier, its active amplifying devices are successively driven between this conductive and non-conductive states during normal operation. In a Class AB amplifier, each active amplifying device is in a non-conductive state for somewhat less than one-half of the total time. In a Class B amplifier, on the other hand, each active amplifying device is in a non-conductive state approximately one-half of the time. in each case, the active amplifying devices are successively driven between conductive and non-conductive states.
As noted above, it has long been recognized that the successive switching of the active amplifying elements between conductive and non-conductive states results in discontinuities and transitional gain variations which are ultimately audible as a distortion generally referred to as cross-over distortion. The discontinuities and gain variations are, of course, also readily detectible and measureable using conventional test procedures and instruments.
The precise mechanism which produces cross-over distortion depends upon the type of active amplifying devices or elements. In the case of amplifiers using bipolar transistors, the discontinuities and transitional gain variations can be explained by consideration of the physical electronics of bipolar transistors. It is well known, for example, that bipolar transistors are essentially current operated devices, and that the base-emitter voltage determines the amount of electronic charge stored at the base-emitter junction due to the effective capacitance of the junction. When a bipolar transistor is driven into the non-conductive state, an increasing amount of charge is stored at the base-emitter junction, depending upon the magnitude of the base-emitter voltage. Thus, when current provided to base of the transistor seeks to drive the transistor back to a conductive state, the effect of the charge stored at the base-emitter junction must first be overcome before true linear amplification can again be achieved when the transistor is restored to the conductive state. This produces discontinuities and so-called transitional gain variations which lead to cross-over distortion. Similar effects occur in amplifiers using field effect transistors (FETs) or vacuum tubes, both of which types of devices are essentially voltage operated devices. With an FET, electronic charge store also occurs at the semi-conductor junctions when the device is driven to a non-conductive state. Transitional gain variations and discontinuities are also experienced when vacuum tubes are driven successively between conductive and non-conductive states.
Others have recognized these problems and proposed various solutions. For example, U.S. Pat. No. 3,995,228 to Pass discloses an active bias circuit which essentially increases or alters the bias provided to the active amplifying elements based upon the instantaneous demand created by the input signal to be amplified. The bias is varied to prevent the active amplifying devices from transitioning to their non-conductive states. Although Pass' approach offers advantages relative to the prior art, the active bias circuit itself must be carefully designed to track the variations in the input signal to be amplified. Indeed, Pass' active bias circuit involve separate amplifiers which depend on sensing the base to emitter voltages across the bipolar transistors.
U.S. Pat. No. 3,883,813 to Seikya similarly discloses a low frequency power amplifier said to be adaptable for operation as either a Class AB or Class B amplifier. An active bias circuit is provided to track the input signal and provide a bias current flow through the power transistors to prevent what Seikya refers to a "notching" distortion. U.S. Pat. No. 3,543,173 to Edgerton appears to disclose a Class B amplifier using an emitter follower configuration and provided with an active bias circuit means to maintain current flow over the range of normal input signals. Again, the effectiveness of Edgerton's apparatus depends upon the tracking ability of the active bias circuit. U.S. Pat. No. 4,025,871 to Peil and U.S. Pat. No. 3,686,580 to Van Den Plassche disclose other amplifier circuits including active bias circuit means for preventing bipolar transistor amplifiers from switching to the non-conductive states during normal input signal conditions.
Similarly, U.S. Pat. No. 4,015,212 to Miyata discloses an amplifier using FET transistors and again having an active bias source intended to minimize the cross-over distortion.